The present invention relates to a mass spectrometer and more particularly, to an atmospheric pressure ionization mass spectrometer capable of correctly analyzing a mass of solutes even under an atmosphere containing a large amount of absorptive substances such as organic compounds.
A liquid chromatograph (hereinafter simply called "LC") is a means for separating mixtures with excellent results, but it has a very poor capability of identifying compounds, i.e., a very poor qualitative performance. On the other hand, a mass spectrometer (hereinafter simply called "MS") has an excellent qualitative performance, but it cannot be used in analyzing mixtures. It becomes possible to analyze mixtures if LC and MS are directly coupled together. However, since MS analyzes samples in vacuum conditions, liquids directly introduced to MS cannot undergo the analysis, thus necessiating an interface between LC and MS. Atmospheric Pressure Ionization (API) has been proposed heretofore as such an interface. Examples of an atmospheric pressure ionization mass spectrometer using API are shown, for example, in No. JP-B-57-25944; Thomas R. Covey et al., "Liquid Chromatography/Mass Spectrometry", ANALYTICAL CHEMISTRY, Vol. 58, No. 14, Dec. 1986, p. 1456; and E. C. Horning et al., "Liquid Chromatograph-Mass Spectrometer-Computer Analytical Systems, A Continuous-Flow System Based on Atmospheric Pressure Ionization Mass Spectrometry", Journal of chromatography, 99, 1974, 13-21, p. 15.
FIG. 1 is a block diagram of a liquid chromatograph-mass spectrometer (LC/MS) analytical system, corresponding to that shown in FIG. 1, p. 15 of "Journal of Chromatography", to which the present invention is applied.
As shown in FIG. 1, an effluent of mobile phase and solutes from LC 1 is introduced to an LC/MS interface 2 wherein the effluent is first vaporized at a spray/vapor chamber 3 and directed to an ion source unit 4. The molecular solutes are ionized at the ion source unit 4, and introduced to an ion analysis unit 5 made of a mass spectrometer (MS) to be mass analyzed. The ion source unit 4 and the ion analysis unit 5 constitute an atmospheric pressure ionization mass spectrometer one example thereof described in No. JP-B-57-25944 being shown in FIG. 2.
Referring to FIG. 2, reference numeral 11 denotes an inlet for sample gas, 12 an ionization section, 13 a molecular ion reaction chamber, 5 an ion analysis unit, 15 a corona discharge needle electrode for ionizing sample gas, 17 a secondary electron multiplier for use in detecting ions, 18 an auxiliary electrode having a first small aperture 20a (e.g., 200 .mu.m in diameter), 21 an electron gun, 22 an electron beam control electrode, 23 an ion attraction and acceleration electrode, 24 an ion lens electrode, 14 a variable DC voltage source with an output of 4 to 5 kV, 16 a quadrupole, 26 a DC amplifier, and 28 a data processor.
The molecular ion reaction chamber 13 is connected to a vacuum pump (not shown) and also acts as a room for differential pumping.
The electron gun 21 is used for electron bombardment for the calibration of ion mass.
Using the above atmospheric pressure ionization mass spectrometer will be described, by way of example, how organic compounds contained in nitrogen gas are analyzed at an atmospheric pressure. Corona discharge between the corona discharge needle electrode 15 and the auxiliary electrode 18 within the ionization unit 12 causes nitrogen gas to be ionized into nitrogen ions which are introduced into the molecular ion reaction chamber 13. In the molecular ion reaction chamber 13, nitrogen ions are subjected to molecular ion reaction with a minute amount of water in the order of several ppm contained in nitrogen gas, to thereby produce H.sub.3 O.sup.+ ions which are subjected to molecular ion reaction with organic compounds likely to be ionized and contained in nitrogen gas to ionize them. A small amount of organic compound ions thus produced are introduced via the small aperture 20a into the ion analysis unit 5 to be separated and analyzed.
Conventional atmospheric pressure mass spectrometers of this type have an excellent sensitivity in detecting a minute amount of components contained in a gas, but they cannot be used in analyzing a specimen containing a large amount of absorptive substances such as organic compounds. The reason for this is that a large amount of organic compounds contained in the gas are deposited on the needle electrode generating corona discharge, and are changed in to macromolecule compounds of insulative nature, thereby leading to unstable discharge. Namely, deposition of insulative macromolecule compounds on the needle electrode decreases a corona current and reduces ion current output. In the meantime, the potential of the needle electrode rises to ultimately result in dielectric breakdown and an abrupt rise in ion current output.
Therefore, as corona discharge becomes unstable, ion current output also becomes unstable so that it is difficult to stably conduct chromatography and mass spectrometry, with the essential high sensitivity of API impaired.
Different from the ionization of impurities in pure gas, an LC/MS system transports a liquid in the amount of several ml/minute to several l/minute so that after vaporization it changes to a volume from several l/minute to several ml/minute. Consequently, the needle electrode 15 is likely to be contaminated with organic compounds or the like.
The spectrometer shown in FIG. 3 was used in measuring an output ion current I, i.e., an output current from the DC amplifier 26, using lipids such as monogalactosyl diacyl glycerol as specimen to be analyzed, the measurement result being shown in FIG. 3. As apparent from FIG. 3, the output ion current unstably fluctuated between a range of about .DELTA.I so that, of the main components (1) to (5) of the specimen the components (4) and (5) with a low output ion current, could not be detected.